Resistance spot welding is a process used in a number of industries for joining two or more metal workpieces together. The automotive industry, for example, often uses resistance spot welding to join together pre-fabricated metal workpieces during the manufacture of a vehicle door, hood, trunk lid, or lift gate, among others. Multiple individual resistance spot welds are typically made along a periphery of the metal workpieces or at some other bonding region to ensure the vehicle part is structurally sound. While spot welding has typically been performed to join together certain similarly-composed metal workpieces—such as steel-to-steel and aluminum alloy-to-aluminum alloy—the desire to incorporate lighter weight materials into a vehicle body structure has created interest in joining steel workpieces to aluminum or aluminum alloy (hereafter collectively “aluminum” for brevity) workpieces by resistance spot welding.
Resistance spot welding, in general, relies on the resistance to the flow of an electrical current through contacting metal workpieces and across their faying interface to generate heat. To carry out such a resistance welding process, a pair of opposed welding electrodes are typically clamped at aligned spots on opposite sides of the workpieces at a predetermined weld site. An electrical current is then passed through the workpieces from one welding electrode to the other. Resistance to the flow of this electrical current generates heat within the workpieces and at their faying interface. When the metal workpieces being welded are a steel workpiece and an aluminum alloy workpiece, the heat generated at the faying interface initiates a molten weld pool in the aluminum alloy workpiece. This molten aluminum alloy weld pool wets the adjacent surface of the steel workpiece and, upon stoppage of the current flow, solidifies into a weld joint.
Resistance spot welding a steel workpiece and an aluminum workpiece together presents certain challenges. These metals have considerable dissimilarities that tend to disrupt the welding process. For one, steel has a relatively high melting point and relatively high thermal and electrical resistivities, while aluminum has a relatively low melting point and relatively low thermal and electrical resistivities. As a result of these differences, aluminum melts more quickly and at a much lower temperature than steel during current flow. Aluminum also cools down more quickly than steel after current flow has ceased. Controlling heat balance between the two metals so that a molten weld pool can be rapidly initiated and solidified in the aluminum workpiece can therefore be challenging. It has been found, for example, that upon rapid cooling, defects in the aluminum workpiece such as shrink porosity or shrinkage, gas porosity, oxide residue, and micro-cracking are drawn toward and tend to gather at the faying interface. Additionally, prolonged heating during resistance spot welding—more specifically an elevated temperature in the steel workpiece due to its relatively higher resistance—is conducive to the growth of brittle Fe—Al intermetallic layers at the faying interface. These two conditions have been shown to reduce the peel strength of the ultimately-formed weld joint and weaken the overall integrity of the established joint between the workpieces.